The invention relates to a process for etching semiconductor wafers in which the wafers are exposed to an etchant in the form of a froth, and to an apparatus for carrying out this process.
Semiconductor wafers, such as silicon wafers, are obtained from a single crystal ingot, such as one grown of silicon, by a process which includes the steps of slicing the ingot in a direction normal to the axis of the ingot to produce a thin wafer, lapping the wafer to planarize its front and back surfaces, etching the wafer to remove any work-damage created by the sawing and lapping and to remove any embedded lapping grit, and polishing the etched surface.
In a typical etching process, a plurality of semiconductor wafers are fixtured in an etch rack or barrel and the entire rack is immersed in an etchant. The etch rack is typically composed of a drum-like casing having one or more parallel, horizontal rollers, each having a plurality of endless circumferential grooves cut in the surface thereof at regular intervals. The grooves are aligned such that a wafer placed in one of the grooves of each of the rollers will stand normal to the axes of the rollers and parallel to other wafers held by the etch rack. The rollers are rotatable about their axes and are driven by a drive mechanism so that each of the wafers is caused to rotate about an axis which is parallel to the axis of the rollers. See, e.g., U.S. Pat. Nos. 3,964,957 and 5,211,794.
Etchants in routine use typically contain a strong oxidizing agent, such as nitric acid, dichromate, or permanganate, a dissolving agent, such as hydrofluoric acid, which dissolves the oxidation product, and a diluent such as acetic acid. The relative proportion of these acids which produces the smoothest and most uniform etching, however, is one at which the removal rate is still relatively high. To minimize nonuniformity, therefore, the wafer rotation speed must be relatively high, e.g., 20 to 30 rpm, to prevent taper from being etched into the wafer. Because the wafers are closely spaced (4 to 7 mm apart), however, any rotation of the wafers tends to produce a rigid-body rotation of the liquid between the wafers and, as a result, the acid between the wafers is relatively stagnant. This rigid-body effect is pronounced at speeds as low as 5 rpm and is problematical at typical rotation speeds of 20 to 30 rpm. This effect, coupled with the large blunt shape of the etch racks or barrels which have been used to date, has led to nonuniform etching across individual wafers and to nonuniform etching along the length of the barrel so that wafers at different positions tend to experience different removals.
In an effort to improve the uniformity of the etching, gases have been sparged into the etch tank and caused to flow between wafers held by the etch rack or barrel. See, e.g., U.S. Pat. Nos. 4,251,317 and 4,840,701. The results, however, have not been entirely satisfactory for several reasons. First, it has proven difficult to produce a uniform flow of gas bubbles across the entire etch rack or barrel as well as across the entire surface of each wafer. Second, the spargers do not tend to produce an approximately equal distribution of small, intermediate and large bubbles which are necessary to produce a wafer which has relatively little microroughness, local thickness variation and total thickness variation; small bubbles (&lt;5 micrometer in diameter) reduce the microroughness of the wafer, intermediate-sized bubbles (5 micrometer to about 2 mm in diameter) preserve low local thickness variation of the wafer, and large bubbles (&gt;2 mm in diameter) preserve low total thickness variation across the diameter of the wafer. Third, because of the high rotation rates typically used, the rigid-body effect has not been entirely eliminated through the use of spargers.